Portable electronic devices, such as portable personal computers, have decreased fairly dramatically in size in the last few years. Early portable computers weighed over twenty pounds and more realistically resembled a desktop computer with a handle--thereby allowing the computer to be lugged around only by the strong and the truly sadistic. In contrast, today's laptop and subnotebook computers can weigh less than three pounds, can easily fit in a briefcase and are truly portable. While the decrease in size of portable personal computers has been a boon to business travelers, it has not been without its problems.
One such problem is that as a computer or other electronic device gets smaller, many of the components contained in the computer or other electronic device must also get smaller. Examples of components that have shrunk along with the device is the circuitry, the hard disk storage, and the speaker. The reduction in the size of the circuitry and the hard disk storage has not been much of a problem, since technological advancements in these areas has allowed equal or better functionality to be present in smaller sizes than what was previously available in the larger sizes.
The reduction in the size of the speaker, on the other hand, has caused more severe problems. As the speaker gets smaller, the maximum power the speaker can handle also gets smaller, thereby reducing the loudness and quality of sound the speaker can produce. In addition, the low frequency response of small speakers must be traded off against the maximum power the speaker can handle because of the physical movement of the speaker's coil at low frequencies. The low power and poor low frequency response of small speakers is wholly unacceptable in today's portable computers and other electronic devices where sound is important, such as when running today's sound intensive multimedia applications.
One prior attempt to solve this problem is disclosed in U.S. Pat. No. 4,727,583 to Weber. This patent discloses a thin speaker contained in a housing in a telephone handset, wherein the speaker cavity is dimensioned to move the resonant frequency of the speaker/housing combination up to approximately the free air cutoff frequency of the speaker. The resulting response then falls off at about 6 db/octave below that point. An amplifier having gain increasing at 6 db/octave is placed in the signal input path for creating an essentially flat frequency response in the normally desired audio range. While Weber's device does appear to increase this low frequency range, it requires extra circuitry to accomplish this result, thereby undesirably increasing the size and cost of the electronic device.
Front to back cancellation of relatively low frequency sound is a fundamental problem of large standalone moving diaphragm loudspeakers. Designers of large standalone loudspeakers normally address this problem by sealing the sound from the back of the speaker in a volume of air. Another common approach follows the early research of James F. Novak, A. N. Thiele, and Richard H. Small, who have theorized since the late 1950s that the operation of large standalone loudspeakers would be enhanced if they provided a vent or port for the phase corrected sounds coming from the back of the loudspeaker.
The following articles are representative of their research:
1. James F. Novak, Performance of Enclosures for Low-Resonance High-Compliance Loudspeakers, IRE Transactions in Audio, Vol. AU-7, pp. 5-13 (Jan-Feb 1959). PA0 2. N. Thiele, Loudspeakers in Vented Boxes: Part I, Journal of the Audio Engineering Society, Vol. 19, No. 5, pp. 382-392 (May 1971). PA0 3. N. Thiele, Loudspeakers in Vented Boxes: Part II, Journal of the Audio Engineering Society, Vol. 19, No. 6, pp. 471-483 (June 1971). PA0 4. Richard H. Small, Simplified Loudspeaker Measurements at Low Frequencies, Journal of the Audio Engineering Society, Vol. 20, No. 1, pp. 28-33 (Jan/Feb 1972). PA0 5. Richard H. Small, Direct-Radiator Loudspeaker System Analysis, Journal of the Audio Engineering Society, Vol. 20, No. 5, pp. 383-395 (June 1972). PA0 6. Richard H. Small, Closed Box Loudspeaker Systems Part I: Analysis, Journal of the Audio Engineering Society, Vol. 20, No. 10, pp. 798-808 (December 1972). PA0 7. Richard H. Small, Closed Box Loudspeaker Systems Part II: Synthesis, Journal of the Audio Engineering Society, Vol. 21, No. 1, pp. 11-18 (Jan/Feb 1973). PA0 8. Richard H. Small, Vented Box Loudspeaker Systems Part I: Small-Signal Analysis, Journal of the Audio Engineering Society, Vol. 21, No. 5, pp. 363-372 (June 1973). PA0 9. Richard H. Small, Vented Box Loudspeaker Systems Part II: Large-Signal Analysis, Journal of the Audio Engineering Society, Vol. 21, No. 6, pp. 438-444 (July/August 1973). PA0 10. Richard H. Small, Vented Box Loudspeaker Systems Part III: Synthesis, Journal of the Audio Engineering Society, Vol. 21, No. 7, pp. 549-554 (September 1973). PA0 11. Richard H. Small, Vented Box Loudspeaker Systems Part IV: Appendices, Journal of the Audio Engineering Society, Vol. 21, No. 8, pp. 635-639 (October 1973).
While the work of these researchers contemplated the practical use of their theories in large, standalone loudspeakers, they failed to contemplate how, or even if, these theories would find practical applicability in small speakers enclosed in a portable electronic device. Likewise, designers of small portable electronic devices have thus far failed to contemplate how, or even if, the theories of these researchers could find practical applicability in small speakers enclosed in a portable electronic device.